A comparative study of the fluorescence of selected flexible and rigidized 7-aminocoumarin derivatives (coumarin 6, 138 and its rigidized coumarin 106) has been made in a series of solvents of varied properties. The studies were further augmented by the quantum chemical SCF-CI MO calculations which provide a theoretical basis for interpreting specific interactions and complex formation with the polar solvents in the intramolecular charge transfer singlet state of these dyes. Moreover, rotational relaxation behaviour, as reflected in the fluorescence polarization of these dyes, is correlated with solvent viscosity/free volume. Through measurements of the temperature/viscosity dependence of the fluorescence depolarization (1/P) in glycerol, we are able to establish an Arrhenius-like relationship linking (1/P) to the free-volume fraction of the medium. Furthermore, depolarization data fitted the Perrin equation with a slope proportional to the sensitivity of the molecular structure towards medium fluidity. The results imply the promising utility of these highly fluorescent dyes as fluorescent probes for local fluidity and polarity of the surrounding medium of interest. We demonstrate possible analytical applications of commercially available coumarin 6 as a promising fluorescent probe for medium properties (e.g. fluidity, polarity and for following micellization in the solutions of some ionic surfactants).

Fluorescence characteristics of two substituted coumarin dyes have been investigated. The effect of solvent polarity and viscosity on the fluorescence properties of these dyes was examined. It was found that the fluorescence quantum yield (φ_f) is a steep function of the viscosity. The dependence of φ_f on viscosity is successfully explained on the basis of the free volume theory of viscosity. Moreover, fluorescence depolarization measurements in media of different viscosities assist in the establishment of an analytical method for microviscosity determination in the interior of some micellar assemblies.

The photophysical properties of some donor-acceptor dye molecules such astrans-4-dimethylamino-4′-cyanostilbene (DCS) and its model bridged single bond derivative (DCSBS) and bridged double bond derivative (DCSBD) with different degrees of flexibility were investigated. This work reports the effect of molecular structure as well as medium viscosity on the fluorescence properties, including fluorescence quantum yields, of this class of molecules. The result could be explained on the basis of a multidimensional excited state which lead to the conclusion thatE/Z isomerization (or double bond twisting) in the excited singlet state is the most important nonradiative deactivation channel to the ground state. Moreover, experimental results obtained by using stationary fluorescence technique reveal the formation of very weak dye-protic solvent exciplexes of different stoichiometries. Furthermore, dynamical relaxation behaviour, as reflected in the fluorescence quantum yield, is correlated with solvent viscosity/free volume. The results point to possible profitable exploitation of this class of molecules as fluorescent probes.

The effect of adding [Ru(bpy)₃]²⁺ complexes during photoelectrochemical etching ofn-silicon on the photocurrent generation is studied. Remarkable enhancement of photocurrent is induced due to the presence of the dye redox system. Redox stabilization of silicon via electron transfer process based on remarkable interfacial interaction between reducing species and the photoelectrode is efficiently achieved. Transient photoelectrochemical measurements during anodic dissolution ofn-silicon in the presence of the complex redox system resulted in inhibition of photocurrent oscillations which was observed at high potentials in the absence of dye redox system. Practically, all photogenerated holes reaching the silicon electrode surface will be efficiently reacted with the coordination redox system resident at the electrode surface. A mechanism of current oscillations, based on periodic buildup and decay (hole/electron recombination) of space charge within the superficial oxide layer is proposed.